In conventional heat pipes, particularly those designed for zero-g operation at temperatures below about 400.degree. F., the thermal transport capacity of the device is controlled by the wick hydrodynamic limit in which the viscous flow pressure losses are equal to the maximum capillary induced pressure difference. A number of techniques have been proposed to eliminate or bypass this hydrodynamic limit; e.g.:
Use of internal conventional or electromagnetic pumps which are electrically powered with the leads penetrating the heat pipe envelope. PA1 Electrostatic pumping using internal high voltage electrodes supplied from an external source of power and using a dielectric working fluid. PA1 Osmotic membranes. PA1 Extending current artery and/or grooved heat pipe technology to provide higher hydrodynamic pumping limits.
The use of a liquid pump, particularly in space radiator applications whose length may approach 60 feet (18 meters), is complicated by the length of the condenser which may extend the length of the space radiator. In such applications, there is no convenient sump. Also, electromagnetic pumping is difficult because the working fluids are dielectrics that must be relatively pure. Using additives in the working fluid to attain a pumping capability generally leads to a concentration buildup in the evaporator that adversely effects performance.
Electrostatic pumping has been the subject of some development work, generally with high voltage electrodes running the length of the heat pipe. The configuration is relatively complex by heat pipe standards and the reliability of the technique is unknown.
Osmotically pumped heat pipes were proposed quite early in heat pipe history but have only recently been subjected to active development. In osmotic pipes, condensate flows through a membrane into a solution which then flows to the evaporator. If the solution path is long and slender, however, mass diffusion within the solution will be negligible at even modest solution flow rates, the solute concentration on the solution side of the membrane becomes extremely low and the osmotic pressure will drop to a very low value. Thus, since long, slender solution flow paths are involved in heat pipes used as space radiators, an osmotic pipe would require some form, for example a liquid pump, of forced solution circulation with attendant disadvantageous complexities.
The approach taken in the subject invention is that of extending current capillary-pumped heat pipe technology to raise the hydrodynamic pumping limit. Either artery or groove approaches can be taken to increase heat pipe performance, but there are believed to be inherent advantages in the groove configuration.
Artery pipes present difficult scale-up problems for validation in 1-g conditions due to artery priming limits. Also, the artery configuration generally results in a greater vapor pressure drop for a given tube diameter and the sensitivity of arteries to gas bubble blockage has been well documented. Finally, the artery will always be a more difficult and costly large scale production process than a groove pipe which, after its forming die is designed and made, can be produced in quantity at low cost.